Re: [PATCH 03/14] cxl/mem: Find device capabilities

[Ben Widawsky]

On 21-02-02 18:10:16, Christoph Hellwig wrote:
> On Fri, Jan 29, 2021 at 04:24:27PM -0800, Ben Widawsky wrote:
> >  #ifndef __CXL_H__
> >  #define __CXL_H__
> >  
> > +#include <linux/bitfield.h>
> > +#include <linux/bitops.h>
> > +#include <linux/io.h>
> > +
> > +#define CXL_SET_FIELD(value, field)                                            \
> > +	({                                                                     \
> > +		WARN_ON(!FIELD_FIT(field##_MASK, value));                      \
> > +		FIELD_PREP(field##_MASK, value);                               \
> > +	})
> > +
> > +#define CXL_GET_FIELD(word, field) FIELD_GET(field##_MASK, word)
> 
> This looks like some massive obsfucation.  What is the intent
> here?
> 

I will drop these. I don't recall why I did this to be honest.

> > +	/* Cap 0001h - CXL_CAP_CAP_ID_DEVICE_STATUS */
> > +	struct {
> > +		void __iomem *regs;
> > +	} status;
> > +
> > +	/* Cap 0002h - CXL_CAP_CAP_ID_PRIMARY_MAILBOX */
> > +	struct {
> > +		void __iomem *regs;
> > +		size_t payload_size;
> > +	} mbox;
> > +
> > +	/* Cap 4000h - CXL_CAP_CAP_ID_MEMDEV */
> > +	struct {
> > +		void __iomem *regs;
> > +	} mem;
> 
> This style looks massively obsfucated.  For one the comments look like
> absolute gibberish, but also what is the point of all these anonymous
> structures?

They're not anonymous, and their names are for the below register functions. The
comments are connected spec reference 'Cap XXXXh' to definitions in cxl.h. I can
articulate that if it helps.

> 
> > +#define cxl_reg(type)                                                          \
> > +	static inline void cxl_write_##type##_reg32(struct cxl_mem *cxlm,      \
> > +						    u32 reg, u32 value)        \
> > +	{                                                                      \
> > +		void __iomem *reg_addr = cxlm->type.regs;                      \
> > +		writel(value, reg_addr + reg);                                 \
> > +	}                                                                      \
> > +	static inline void cxl_write_##type##_reg64(struct cxl_mem *cxlm,      \
> > +						    u32 reg, u64 value)        \
> > +	{                                                                      \
> > +		void __iomem *reg_addr = cxlm->type.regs;                      \
> > +		writeq(value, reg_addr + reg);                                 \
> > +	}                                                                      \
> 
> What is the value add of all this obsfucation over the trivial
> calls to the write*/read* functions, possible with a locally
> declarate "void __iomem *" variable in the callers like in all
> normall drivers?  Except for making the life of the poor soul trying
> to debug this code some time in the future really hard, of course.
> 

The register space for CXL devices is a bit weird since it's all subdivided
under 1 BAR for now. To clearly distinguish over the different subregions, these
helpers exist. It's really easy to mess this up as the developer and I actually
would disagree that it makes debugging quite a bit easier. It also gets more
convoluted when you add the other 2 BARs which also each have their own
subregions.

For example. if my mailbox function does:
cxl_read_status_reg32(cxlm, CXLDEV_MB_CAPS_OFFSET);

instead of:
cxl_read_mbox_reg32(cxlm, CXLDEV_MB_CAPS_OFFSET);

It's easier to spot than:
readl(cxlm->regs + cxlm->status_offset + CXLDEV_MB_CAPS_OFFSET)


> > +	/* 8.2.8.4.3 */
> 
> ????
> 

I had been trying to be consistent with 'CXL2.0 - ' in front of all spec
reference. I obviously missed this one.